Fuel cells electrochemically convert fuels and oxidants to electricity. Fuel cells are employed in many environments, for example, automotive, aerospace, industrial, etc. Unlike a battery, which contains a set amount of chemicals for generating electricity and which stops delivering electricity once the chemicals are consumed, a fuel cell can deliver electricity continuously so long as it receives fuel and oxidant.
Fuel cells are generally categorized according to the type of electrolyte (e.g., solid oxide, molten carbonate, alkaline, phosphoric acid, or solid polymer) used to accommodate ion transfer during operation.
For example, a solid polymer electrochemical fuel cell generally comprises an MEA (membrane electrode assembly). The MEA includes a solid polymer membrane or PEM (proton exchange membrane) sandwiched between and in contact with two electrodes (one called an anode and the other called a cathode) made of porous, electrically conducting sheet material. The electrodes are typically made from carbon fiber paper or cloth. In addition, at the interface of the electrode and membrane, i.e., sandwiched therebetween, is a platinum-based catalyst layer to facilitate the electrochemical reaction.
Typically, the MEA is placed between two electrically conductive graphite plates which have one or more reactant flow passages impressed on the surface. The reactant flow passages direct the flow of a reactant (e.g., fuel or oxidant) to the electrode. Additional cells can be connected together in series to form a fuel cell stack having increased voltage and power output. Such a fuel cell stack is typically provided with inlets, outlets, and manifolds for directing the flow of the reactants (as well as coolant, such as water) to the individual reactant flow plates.
Fuel, such as hydrogen, is supplied to the anode side of the fuel cell where the hydrogen reacts at the platinum-based anode catalyst layer to separate into hydrogen ions and electrons, as follows (anode reaction):
H.sub.2.fwdarw.2H.sup.+ +2e.sup.- PA1 1/2O.sub.2 +2H.sup.+ +2e.sup.-.fwdarw.H.sub.2 O
The solid polymer membrane permits the passage of protons (i.e., H.sup.+ ions) from the anode side of the fuel cell to the cathode side of the fuel cell while preventing passage therethrough of reactant fluids (e.g., hydrogen and air/oxygen gases). The electrons migrate via an external circuit in the form of electricity.
An oxidant, such as oxygen or oxygen containing air, is supplied to the cathode side of the fuel cell where it reacts at the platinum-based cathode catalyst layer with the hydrogen ions that have crossed the membrane and the electrons from the external circuit to form liquid water as a reaction product, as follows (cathode reaction):
Typically, a plurality of fuel cells are assembled between a pair of thick rigid end plates. The edges of the end plates are bolted together to apply a compressive force on the plurality of fuel cells. A problem with a fuel cell assembly having thick rigid end plates is that the thick end plates increase the weight of the fuel cell assembly.
Another problem with such end plates is that they have a tendency to deflect when bolted together so that an unevenly distributed compressive force is applied to the plurality of fuel cells.
One approach has been to assemble a fuel cell stack with one or more end plates having a bladder filled with a gas such as nitrogen or a liquid such as oil to facilitate distribution of forces applied to the plurality of fuel cells. End plates having such a bladder may have a tendency to leak over time thereby reducing the gas or liquid therein. This may result in a reduced stack compression pressure applied to the plurality of fuel cells, reduced power output, and thus may require periodic maintenance.